The invention relates generally to radio frequency receivers, and more particularly to circuits for sampling and converting received bandpass signals to numerically quantified equivalents.
Sampling circuits, consequently, constitute interfaces between front-end analog signal stages and back-end digital signal processor stages of state-of-the-art radio receivers. Digital Signal Processor (DSP) operations have become essential to modern radios, especially in implementing modem functions of these radios. Radio receivers which use digital signal processing techniques include those that use digital modulations, like PSK, as well as those that use analog modulations, like AM and FM. Until recently, all modem functions were performed by analog circuitry.
VLSI technology and the deployment of communications satellites have concurrently spawned a rather rapid development of digital radio communications technology. Digital communications technology includes digital information encoding or modulation, and conversely, decoding or demodulation, of carrier frequencies with signalling techniques known, among others, as phase shift keyed (PSK) or differential phase shift keyed (DPSK) signalling.
The basic processing techniques to convert an RF analog signal received at an antenna, through frequency downconversion, to a digital baseband signal are generally well understood and used. However, the techniques used in the existing art for downconversion and sampling functions generally demand comparatively costly components. In order for digital communications to penetrate large consumer market segments, as would be desirable for mobile satellite and digital cellular services, innovations in the radio art are required not only to improve performance, as has been the trend in the past, but also to reduce the cost of radios.
It is therefore highly desirable to implement known signal digitization and down conversion processes in more economical and more readily manufacturable apparatus than has been provided by the complex prior art apparatus and methods.